The present invention relates generally to cables for use in a downhole environment, such as may be used in oil or gas wells for conveying well logging tools and other types of equipment within wellbores, and as may be otherwise be used for communication with devices located in downhole environments.
Many types of cables have been used over the years for communication with logging tools and other equipment located in a downhole environment. The most common of these cables are typically referred to as “wireline,” by virtue of their inclusion of one or multiple layers of wire armor which also serve as the load bearing members of the cable. While wireline cables are typically durable, at least in many environments, they are heavy and not always well-suited for certain applications.
For example, in many high-pressure environments obtaining adequate pressure sealing around a wireline cable can not only be difficult, but can have environmental consequences. For example, one method of establishing sealing in such a high-pressure environment comprises a high-pressure pack off which injects grease under high pressure to provide the necessary sealing between various types of pack off stuffing elements and the non-uniform surface of the wireline. However, such systems create a great deal of friction that can impede movement of the cable. Additionally, the injected grease can often present an environmental hazard, such as when it is introduced to the surface environment, such as when the wireline is removed from a wellbore. Also, in some cases the weight of the wireline and the friction involved in high pressure operations presents a barrier to the depth to which the cable and attached tool strings may be deployed, particularly in high pressure environments.
Because of these difficulties with wireline, cables have been proposed to minimize the problems associated with a non-uniform external surface, and also to reduce the weight of cables. While these proposed cables are believed to achieve some advantages over wireline-type cables, they are not perfect for all applications. For example, in such proposed cables the load bearing capability is typically provided by polymer fibers, such as fibers of the polymer marketed under the trade name Zylon (believed to be a trademark of the Toyobo Corporation). Zylon is understood to be a range of thermoset polyurethane synthetic polymers, derived from electron beam cross-linked thermoplastic polyurethane. While Zylon fibers are believed to generally maintain their strength at relatively high temperatures, up to approximately 500° F., and are believed to function adequately in high humidity environments; the current expectation is that such fibers are not compatible with environments that present both high temperature and high humidity. Thus, many high temperature subsurface applications are expected to be problematic for cables utilizing Zylon fibers for the load bearing capability of the cable.
Additionally, many types of corrosive materials commonly found in downhole environments, such as H2S and CO2 are believed to adversely affect Zylon's load bearing capabilities at downhole temperatures. In most conventionally proposed cables, the Zylon fibers are next to the outermost layer. Accordingly, any damage to that outermost layer will allow corrosive liquids or gases, to directly contact the Zylon fibers thereby leading to potential degrading of the fibers. Additionally, any damage in such an outermost layer would typically introduce water to the Zylon fibers, further potentially degrading the fibers. Such cables have been proposed that would include a PETP tape layer between the outer covering and the Zylon fibers; however such tape layers are not known to offer resistance to penetration by the problematic water or the corrosive gases or fluids. Accordingly, conventionally proposed synthetic fiber cables are believed to provide less than optimal capabilities for use in many types of downhole operations.
Accordingly, the present invention provides for new cable structures that are believed to overcome the deficiencies of currently known cable configurations.